phone randomly keeps going out of 4g mode?

i tried searching online and in the forum for this before posting. Anyways, I live right outside of the DC area and have full 4g coverage. Recently, when i turn 4g on it says that I have full 4g signal for about 30 seconds...then the 4g mode will shutoff then go back to normal 3g ...any ideas why this is happening anyone i haven't rooted my phone or don't anything special besides install normal applications!

i tried searching online and in the forum for this before posting. Anyways, I live right outside of the DC area and have full 4g coverage. Recently, when i turn 4g on it says that I have full 4g signal for about 30 seconds...then the 4g mode will shutoff then go back to normal 3g ...any ideas why this is happening anyone i haven't rooted my phone or don't anything special besides install normal applications!